The Neoichnology of the Modern Fluvial and Estuarine Sediments in Moreton Bay, Queensland, Australia: Relationships of Terrestrial, Freshwater and Marine Organisms to Physicochemical Characters of Sedimentary Systems

Abstract

The study of modern sedimentary systems and their biotic and abiotic characters are important to produce ichnologic proxies to interpret depositional environments, their biophysicochemical characters, and their variations through short- and long-term timeframes. Research has been conducted on rivers and estuaries of Moreton Bay, however, little work has focused on the neoichnology and its implications to interpreting deep time deposits. Here we provide examples of how the neoichnology reflects the physicochemical conditions in the southern part of this back-barrier system. Moreton Bay contains various depositional environments, bed forms, and geobodies produced by fluvial, tidal, and wave processes. Despite the fluvial input, marine trace-making organisms dominate most of the Moreton Bay ichnology, with the exception of vegetated islands and strandlines that compose the back-barrier islands. Portions of the mouth bar of Logan River are subaerially exposed at low tide. This mud-dominated, coarsening upward succession of fine- to medium-grained sand is highly bioturbated by such necktic and epi- and endobenthic organisms as callianassid shrimp, solider crabs, polychaete worms, a variety of gastropods and bivalves, stingrays, and fishes. This activity produces such trace fossils as Ophiomorpha, Taenidium, Skolithos, Thalassinoides, Planolites, Palaeophycus, Taphrahelminthopsis, Helminthopsis, Lockeia, and large- and small-scale Piscichnus. These traces are overprinted by anisodactyl, incumbent anisodactyl, semipalmate, and palmate bird tracks with associated feeding marks. Such vegetated islands as Kangaroo Island in the Tiger Mullet estuary are dominated by marine crab burrows along the island margins, as well as inland in low-lying areas; mangrove vegetation shows the same pattern. Inland, the island contains grasses and conifers. Root patterns are extremely shallow and nearly horizontal due to the shallow water table. Subaerial sediments experience pedogenesis mediated by solitary, gregarious, and social insect bioturbation, with gleyed- and redoximorphic-mottled patterns associated with incipient peds and crab burrow walls. Gold bank, the eastern tip of Kangaroo Island, contains similar biota to the Logan River mouth bar, but is dominated by soldier and fiddler crabs and stingray feeding pits, which thoroughly bioturbate the fine-grained sand, except where tidal and wave energy rework the sediment.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015